Television scanner



F. SCHRTER TELEVIS ON SCANNER Filed July 25, 1935 April l 1, 1939.

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Patented Apr. 1l, 1939 UNITED sra'ras PATENT ori-ics TELEVISION SCANNERtion of application July 23, 1935, serial No. 32,768

Germany July 14, 1934 2 Claims.

In the so-called'iconoscope, the television picture to be transmitted isprojected on a surface, consisting' of a mosaic of minute photoelectriccells. Each of these small cells represents a min- 5- iatiue condenserwhich stores the energy of the photoelectric' electron emission over aperiod of time; approximating the duration of the scanning of thepicture, for instance 3&5" second. The charges thereby accumulated, are,for the purpose of modulating the transmitter, scanned by means of acathode ray, passing in a parallel line movement over the mosaicsurface, and moving in synchronism with that in the Brauns tube ofthesuiiicient length of time, the luminous sensitivity,

essentially present in the iconoscope but only of short durationtherein.

The invention makes use of the so-called electron microscope, i. e., thepossibility of highly enlarged reproduction on a. luminous screen of anelectron source contained in high` vacuum, by

means of electrostatic or magnetic lenses. In the present case, theelectron source is represented by a photoelectric cathode I as shown inthe drawing, on which the picture to be transmitted or a portionthereof, preferably an entire picture line 2, is projected in suitablereduction by means of an optical means 3. In the subsequent description,only the last mentioned case will be further considered, i. e., only asingle line acts on the photo-cathode. In decomposing a lm, the linealternation is obtained by the continuous progression oi the film, inscanning a two-dimensional picture iield which is statlonaryin space,this is done by interposing an optical means supplying the secondcomponent of the decomposition, for instance a prismatic reflectorwheel. At the place where the photo-cathode I is impinged by the lightof the picture line projection, there occurs, along the cathode, aphotoelectric emission, varying as regards density, in accordance withthe distribution of the brilliancy, the linear course of said emissionbeing reproduced in eularged size on a separate scanning devicel bymeans of the electron microscope (electrodes 4, 5, 6). The scanningdevice consists of a line of individual microscopic electrodes 8,insulated from (Cl. TIS-7.2)

each other, and arranged within a highly evacuated tube, the saidelectrodes being impinged and charged by the accelerated photoelectrons.These individual electrodes 8 therefore` receive different charge inaccordance with the course of the in- 5 tensity in the picture line. Thecapacity thereof, relative to a common positive collecting electrode 9,can be utilized for storing, analogous to the iconoscope, so that in thecase herein referred to, the accumulation of the luminous effect takes1o place throughout the period of transmission of a picture lineresulting in a large gain of luminous sensitivity, in comparison withthe hitherto applied principle of scanning the television picture frompoint to point. The increase of the energy l5 of the photoelectronscaused in the electron microscope due to the electrical acceleration isutilized in the Widest degree, in accordance with the invention, sinceit is possible in this manner to charge the individual electrodes 8 to ahigh nega- 20 tive potential relative to the opposite electrode 9.

In order to exploit the varying accumulated charges along the contactline l, for the modulation of the television transmitter, an independentconstant cathode ray having a sufficiently 25 small cross section ofincidence, is used. This cathode ray, indicated by the dotted line I0,is derived from an independent source (glow cathode I I) and is movedalong 'I in synchronism and equal phase with the line recording ray ofthe 30 image receiving tube. Since it is hereby aimed at neutralizingthe more or less high negative charges of the individual condensers 8,it is necessary to liberate from the incident ray I0, a. correspondingpositive charge. 'I'his will advanta- 35 geously be accomplished bymeans of secondary electron emission, released by impinging or primaryelectrons on the electrodes 8 of the contact line, or preferably onseparate contact surfaces I2 individually connected to the saidelectrodes. 40 The secondary emission can be enhanced bycovering thesurfaces l2 with coatings favoring electron emission or forming saidsurfaces of suitable metal; furthermore by providing suitable velocitiesof particles in the scanning cathode ray, an'd 4,5 placing opposite therepulsing faces I2 a collecting electrode I3, having a relatively highpositive potential, and whose iield makes possible the withdrawal by wayof suction of the secondary electrons. On the contrary, in the electronmi- 50 croscopic reproduction of the photoelectron source I, adetrimental secondary emission of this contact series towards theelectrodes 8 can be prevented by means of-a highly negative co1- lectingelectrode I4 placed opposite the electrodes 5g 8 and acting like asuppressor grid of a pentode. The high secondary emission at thescanning surfaces? I2 has the effect that the latter emit during thetime unit, a greater number of electrons than was supplied thereto bythe ray I0. In other words, positive charge will be liberated forcompensating the negative charge of the electrodes 8, the compensatingeffect to be utilized for modulating the transmitter varies, as desired,in its intensity in accordance with the distribution of brilliancy alongthe picture line.

By interrupting, at scanning, the cathode ray I by known means(auxiliary electrode near the cathode II) in the rhythm of a highfrequency whose duration of the cycle be a Whole number fraction of theduration of the traversing of the individual surface elements I2 (inorder to avoid disturbing interference between the period ofinterruption and the screen of the contact line), there will bereadilyobtained the carrier oscillation desired for further intensifyingthe picture signals.

An advantage of the described arrangement resides in that thephotoelectrically effective surface I is not continuously impinged byrapid electrons and thus disactivated as in the case of the iconoscope.Positive ions arrive in excess at the highly negative intermediateelectrode 5 protecting the photo-cathode I against rapid attack. If, aspointed out, only a single picture line is reproduced on i it is easy tofind thereon a zone of suiciently uniform photoelectric sensitivity.Furthermore, it would be possible to displace the picture line on thesurface i for instance in vmaking it displaceable or rotatable from theoutside, in usin'g for example a rotating' surface, or it is providedwith a very fine screen of sharp edges on which an especially high anduniform photoelectron emission appears.

A further advantage of the arrangement according to the inventionresides in the fact that it is not necessary to pass separate,insulating lead-ins from the always numerous individual elements 8 ofthe contact line 1, through the wall of, the tube, and which wouldotherwise render the manufacture thereof extremely difficult. If thephoto-cathode I is to be utilized for the reproduction of larger pictureportions, the linear contact series 'I must be substituted by acorresponding two dimensional screen having a very large number of-individual electrodes. In this case, the common positive opposing layer9 is formed as a fine net, the scanning ray I 0 passing through themeshes thereof, While the individual electrodes 8 are formed-as separateconductors, passed through an insulating carrier layer (for instancemica) and ending on the rear side into points standing freely in themeshes of the net 9. When the cathode ray I0 impinges onthese points,the desired secondary emission occurs thereon by means of which theindividual charges of the electrodes 8 are momentarily destrayed. Thesame steps may also be taken, as

is obvious, where 1 represents a single 'contactl line, as abovereferred to.

'I'he compensating current impulses produced at the discharge of thesingle elements 8--9 of the contact line or of the contact screen, actacross a coupling resistance I5, on the input tube I8 of the imageamplier I'I. By means of a suitable circuit care is to be taken that thecoupling resistance I5 is passed only by the desired current impulses ofthe picture point scanning and which are defined by the frequencythereof, while the sum of the lpositive charging currents, released in 9by the photoelectrons, and whose period is given by the picture linetransmission period, is rendered ineffective with respect to themodulation, by the introduction of 'a frequency independent shunt, or inan equivalent manner.

The surfaces 8 impinged by the photoelectrons may themselves be utilizedfor the scanning by means of the cathode ray IB, the latter thenarriving from the front side, and to this end a common auxiliaryelectrode will be placed opposite the said surfaces, which during the incidence of the photoelectrons is maintained predominantly negative,while during the incidence of the scanning cathode ray it is maintainedhighly positive for the purpose of withdrawing by way of suction, thesecondary electrons. In

this case it appears however necessary to limit the illumination to afraction for instance 10% of the transmission period of each line, inorder to permit scanning by I 0 during the remaining period of time.

Having thus described the invention, what is claimed and desired tosecure by Letters Patent is the following:-

1. In a television transmitter, an evacuated envelope havingV at leastone elongated section containing a photoelectric cathode, meansresponsive to the optical image of the object to be transmitted,reflecting means embracing at least a part of the elongated section ofsaid envelope for directing the image on to said photoelectric means anddeveloping a current image thereof, a double sided photoelectric mosaiccontaining a plurality Iof discrete photoelectric elements on the sidethereof facing said photoelectric cathode, means for focusing thecurrent image on to said photoelectric mosaic whereby a charge image isformed on said mosaic corresponding to the current image, means f ordeveloping a cathode ray beam, means for deiiecting the said beam in atleast one coordinate, and means for directing said beam to said doublesided mosaic on the side thereof remote from said photoelectric meanswhereby said beam neutralizes the charge image formed on thephotoelectric section of the mosaic.

2. Apparatus in accordance with claim 1 wherein said photoelectricmosaic containing said discrete photoelectric elements comprises asingle row of said elements corresponding to one linear section of theoptical image to be transmitted.

FRITZ SGHRTER.

